Klebsiella pneumoniae (KP) are increasingly important Gram-negative bacterial pathogens that cause pneumonia, wound and urinary tract infections, bacteremia and meningitis. Like other Enterobacteriaceae, KP have become resistant to extended spectrum of beta lactamases (ESBL) antibiotics, including third generation cephalosporins. KP now comprise an increasing percentage of these ESBL-producing bacteria and alarmingly, in the last decade KP have become increasingly resistant to carbapenem. Carbapenamse-resistant KP (KPC) now represent 8% of all KP and in data reported to the CDC have increased over 6 fold in the last decade, resulting in extended hospitalization, increased costs and mortality. These nightmare bugs necessitate the use of toxic, less effective last resort antibiotics. Based on the dwindling pipeline of antibiotics being developed, there is little likelihood that new antibiotics will be available in the near term. A vaccine against KP could elict antibodies that would provide adjunctive therapy for KP that would not be subject to antibiotic resistance mechanisms. We previously developed a 23-valent KP capsular polysaccharide vaccine that progressed to human trials. Since relatively few KP O serotypes account for >70% of KP infections, we reason that a multivalent COPS-based KP vaccine could provide wide coverage and be less difficult and expensive to produce. We propose to develop a prototype conjugate vaccine against the core/O polysaccharide from the lipopolysaccharide (LPS) of a virulent KP (O1K2) linked to an established carrier protein, CRM197 by two different conjugation chemistries. In Specific Aim 1 we will construct a recombinant attenuated strain of KP that can serve as a reagent strain that can be used to safely purify large amounts of KP COPS. As we have successfully done for other GNB pathogens (Shigella, Salmonella, Francisella) we will generate mutations in the guanosine biosynthesis pathway enzymes, and confirm loss of virulence in murine challenge studies. We will generate a second mutation in the genes encoding the KP capsule which will permit a more efficient purification of the surface COPS antigen. In Specific Aim 2 we will test the hypothesis that conjugation of KP type O1 COPS to an established protein carrier, CRM197 will induce anti-LPS antibodies that mediate opsonophagocytosis of KP by phagocytes and will protect mice from lethal homologous KP challenges in both intraperitoneal and intratracheal mouse challenge models. We will vary the linkage (random coupling versus end-linkage) and use of linkers to assess which construct elicits optimal antibody responses and protection. At the conclusion of these studies, we intend to have a KP COPS conjugate vaccine that may serve as a prototype for a multivalent KP COPS vaccine that can be used in the prevention and/or development of a KP-antibody-enriched IVIG for the treatment of KP infection, even if the strains are highly resistant to antibiotics. We also will have a production strain of KP that will be safe for scale-up and allow for more efficient purification of the KP COPS.